The invention relates to a method and device for coating a size-press roll, paper or board or a corresponding moving base, comprising a revolving coating bar which rests against a moving base extending across the machine width. The coating bar is supported in a cradle substantially over its entire length, and is arranged to spread and to smooth a coating agent onto the moving base. The coating agent is introduced into the coating device and onto the moving base in advance of the coating bar in the running direction of the moving base.
The invention also relates to a coating device for coating a size-press roll, paper or board or a corresponding moving base, comprising a coating-agent chamber, which is defined by a revolving coating bar, a front wall of the coating-agent chamber, lateral seals of the coating device, and the moving base. A coating agent is fed into the coating-agent chamber under pressure to thereby provide a pressurized coating-agent chamber. The coating bar is supported on, i.e., rests against, the moving base, and functions as the coating member. The coating bar extends across the width of the machine to thereby coat the entire width of the moving base.
The present invention further relates to a method for coating a size-press roll, paper or board or a corresponding moving base.
At present, in the coating processes for coating paper or board, two alternative methods and devices are commonly used, i.e., a blade coating technique or a bar coater technique. The present invention provides an improvement on the latter bar coaters, which have generally proved excellent especially in film size press techniques. The material commonly used in coating bars currently in use is usually steel. In order to increase the service life of the bar, the bars are provided with chromium plating.
In surface sizing techniques of paper, the face of the bar is provided with grooves, or steel wire may be wound onto the bar to form a texture similar to grooves on the bar face. The use of a grooved bar for surface sizing of paper is based on the fact that the thickness of the size film applied onto the base being coated is determined by means of the depth of the grooves.
It is a very significant drawback of grooved bars known in the prior art that they are prone to wearing quite rapidly. This wear reduces the service life of the bars and causes undesirable standstills to replace the worn bars.
Another drawback of prior art coating bars is that pigmenting with a high dry solids content is also entirely impossible with grooved bars, because the wear of the bars is excessive when used in conjunction with a pigmenting process.
The diameters of the coating bars currently in use are relatively small, generally about 10 mm. This specific diameter is representative of a small-diameter coating bar. Besides the wear of the coating bar, it is a further significant detrimental factor with the use of small-diameter bars that, especially in size-press operation, the roll faces are scratched excessively. The scratching results from the characteristic that the nip between the coating bar and the roll coating is quite open (not sufficiently wedge-shaped). Therefore, contaminants from the paper or size cannot be easily passed through this open nip. These contaminants, when remaining in front of the nip, form grooves in the roll coating and, at the same time, also cause the bar to wear locally.
Further, in the operation of size-presses, it is preferable to use roll coatings softer than normal, i.e., having a hardness of an order of about 35 P&J. In such a case, the contaminants have better access through the nip between the coating bar and the roll coating without producing scratches in the roll coating. This results from the fact that the contaminants are pressed into the soft coating.
When small-diameter bars are used in a bar coating technique, it is a further significant drawback that, when the bars are handled, they tend to undergo permanent distortions, after which they cannot be used any more. Likewise, web breaks frequently also bend and twist the bars permanently rendering the bars unusable.
In bar coaters utilizing large-diameter bars, these problems have not occurred when such bars are provided with a smooth-face for use in pigment-coating operation.
With respect to the prior art, reference is made further to Finnish Patent No. 30,147.